Push-pull amplifier



Jan. 4, 1944. v. M. cousms PUSH-PULL AMPLIFIER Filed May 13, 1942 F/G/H) AME 1.040

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e R- m AMP. LOAD INVENTOR lZMCOUS/NS ATTORNQQ Patented Jan. 4, 1944UNITED STATES PATENT OFFICE,

PUSH-PULL AMPLIFIER Van M. Cousins, Chatham, N. .L, assignor to BellTelephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application May 13, 1942, SerialNo. 442,781

2 Claims. (Cl. 179-171) ing applied signal voltage, a decreasingpropor-- tion of the total signal voltage is efiective at the grid andnon-linear distortion results. Further non-linear distortion isintroduced in other ways such as by variations in. the mu of the tubeand in the impedance of the signal source.

The object of this invention is to reduce this non-linear distortion.

According to the invention rectifier elements are associated with theinputs of the tubes in such a manner that as the signal voltage variesthe proportion of the total voltage which is efiective at the grid isvaried to compensate for the non-linear distortion due to one or more ofthese causes.

In the preferred embodiment of the invention as applied to amplifiers ofthe class B or class C types, the mid-point of the signal source is leftfloating with respect to ground and the input electrodes of each tubeare shunted by a rectifier unit poled to conduct toward the grid. Whenthe signal potential is positive at the grid of one tube, the rectifierunit associated with the other tube is conductive and as the gridcurrent in the active tube increases the resistance of the conductiverectifier decreases to increase the proportion of the generatedsignalvoltage across the input of the active tube. By a proper choice ofthe type of rectifier unit used or by suitably modifying itsnon-linearity by means of associated resistors, the proportion of thegenerated voltage which is effective at the grid may be kept constant orvaried in whatever manner is necessary to compensate also for thenon-linear distortion due to factors other than the variations in thegrid impedance. 1

It will be noted that since the impedance of the rectifier units intheir conducting direction may be much less than the grid impedance ofthe tubes, a large proportion of the total signal voltage may be appliedalternately to the tubes thereby giving, in addition to the'reduction ofto use the mid-point connection to the signal source, compensation fornon-linear distortion may be obtained by connecting one of the rectifierunits in series with each grid connection to the signal source. Therectifier units or the tube input circuits, or both, may be shunted bysuitable resistors for adjusting the non-linearity of the network tosuit the requirements of the particular case.

In the drawing, Fig. 1 shows a system including an amplifier accordingto the invention, Fig. 2 shows the .use of resistors with the rectifierelements to compensate the distortion more accurately and Fig. 3 showsthe use of distortion correcting elements in a conventionally balancedcircuit.

In Fig. 1 signals from a source I are amplified in the amplifier 2 andimpressed on the grids 3 and 4 of thetubes 5 and 6 by means of anysuitable coupling elements such as a transformer 'I' having a secondarywinding the mid-point of which is left floating with respect to ground.

The tubes 5 and 6 are preferably of the type in which the plate currentis cut off, or very small, at zero grid bias. The plates and cathodesare connected by means of a source of power 9 and an output transformerIII to a load I l. The cathodes are grounded at I! independently of thewinding 8 and the tube inputs are shunted by high resistances l3 and I4and rectifiers l5 and I6 which are oppositely poled to conduct fromcathode to grid in each case.

When the signal potential is positive at the grid 3, the rectifier I6 isa relatively low impedance shunt across the input of the tube 6 and thegreater part of the full potential of the winding 8 is impressed on thetube 5 to drive the grid 3 positive with respect to the cathode of thetube and produce a large increase in the plate current in accordancewith the signal. Similarly, when the signal potential is positive at thegrid 4 of tube 6, rectifier I5 is a relatively low impedance.

across the input of the tube 5 and the greater part of the fullpotential of the winding 8 is impressed on resistor 4 and the platecurrent of the tube 6 is correspondingly increased.

Since the grid-cathode resistance Rg of a tube operating with a positivegrid is a function of the grid voltage and decreases as the grid voltageincreases and the impedance of the signal source is ordinarily morenearly constant, the proportion of the generated voltage appearingbetween the grid and' cathode of the tube will decrease as the signallevel increases thus causing the generation of non-linear distortionproducts.

If in Fig. 1 the rectifler units I! and ii are of the type havingappreciable initial resistance which decreases as the current throughthem increases, the non-linearity of these units will tend to correctfor the variable grid impedanc of the tubes. For example, when apositive signal pulse is applied to grid 3 and the unit It becomesconducting, the grid-cathode impedance decreases as the flow of gridcurrent increases so that the grid-cathode voltage is less than requiredfor distortionless amplification. However, as the current through theunit I6 increases, its impedance decreases thereby increasing theproportion of the total signal voltage applied to the tube 5. Similarlywhen the'grid 4 i driven positive the impedance rectifier unit i5decreases and tends to hold up the grid-cathode voltage of the tube 6.

It will be apparent that for maximum correction of distortion therectifier unit must have a relatively high initial resistance, whereasthe full signal potential is applied to the positively driven tube onlywhen the impedance of the conducting rectifier unit is relatively verylow at all signal voltages. Copper oxide units or their equivalent aretherefore more suitable for di tortion correction and electronic tuberectifiers or their equivalent are better adapted for cases whereincreased gain is th primary consideration.

In Fig. 2 if the total grid impedances R+ and R- are equal at all signalvoltages, non-linear distortion due to grid current will not occurprovided the impedance R0 of the signal source is negligible. Inpractice this is rarely the case and hence the impedance of theconducting rectifier unit and associated modifying resistances mustdecrease faster than the grid impedance of the positively driven tube inorder to compensate for the loss of signal potential inRo' and maintainthe grid-cathode voltage at its proper value throughout the signalcycle.

The non-linearity of the resistance-current characteristic of copperoxide units follows approximately the inverse square law whereas thenon-linearity of the input resistance characteristic of the tubes withgrid current is usually of a lower order so that with such units, ortheir equivalent, it is possible to over-compensate the tube distortionto the extent necessary to correct for the loss of signal potential dueto other causes such as the impedance of the source. In cases where therectifier units alone, or these units with the addition of resistors l3and I4, do not give the required degree of non-linearity, their effectmay be modified, or further modified, as

required by means of shunt resistors l1, I! or series resistors I 8, 18or both. It will be understood however that these resistors should bekept as small as other considerations permit in order to obtain as muchvoltage as possibleat the positively driven grid and thereby reduce thepower capacity required in the driver stage.

The correct values of the modifying resistors for optimum compensationwill depend on the characteristic of the rectifier units used and thevarious constants and characteristics of the particular circuit in whichthey are used. In general it will be understood that increasing thevalue of resistors l 8 (or decreasing the value of resistors l3 and M orH) make the combined resistance or the elements associated with theinput circuits less dependent on the signal voltage and increases thevoltage required to produceia given degree of corrective non-linearity.The optimum resistance values will therefore be determined empiricallyin each case. In cases where correction is being applied to compensateonly distortion arising from non-linearity not intimately associatedwith the input circuit, the phase of the distortion may be such as torequire that the rectifiers be poled in the direction opposite to thatshown in the drawing.

The non-linear distortion correcting feature of the invention may alsobe used in conventionally balanced input circuits as shown in Fig. 3. Inthis case the mid-point IQ of thetranstormer 20 is grounded and therectifier units shunted by suitable modifying resistors 2|, 22 areserially connected in the leads from the signal source to the grids 3and 4. tubes may be shunted by high resistors 23, 24 and the rectifiersare poled to conduct toward the grids as in the circuit previouslydescribed. In the absence of grid current the resistance of therectifiers is relatively high but when the positive signal potential ateither grid becomes great enough to produce grid current the resistanceof the rectifier decreases a the grid current increases. By properchoice of the series and shunt modifying resistors, the non-linearity ofthe network may be adjusted to compensate for both the non-linearity ofthe tubes and the loss of potential in the driving source impedance sothat power is delivered to the load substantially free of non-lineardistortion.

While the circuit of Fig. 3 is satisfactory from the standpoint ofnon-linear distortion, only somewhat less than one half the signalpotential ,is effective at each tube and for this reason the circuits ofFigs. 1 and 2 will ordinarily be preferred since they require less powercapacity in the signal source or driving amplifier.

What is claimed is:

In an amplifying system, two vacuum tubes having inputand outputelectrodes connected in balanced relation, an ungrounded source 01'signals connected to the input electrodes and a network associated withthe input electrodes of each tube including a rectifier element having anonlinear resistance current characteristic for compensating fornon-linear distortion produced by.

the impedance of the source and by the nonlinearity of the impedance ofth tubes when current fiows between the input electrodes of the tubes.

' 2. In an amplifier comprising two vacuum tubes having grid and cathodeinput electrodes and output electrodes connected to form-balanced inputand output circuits, said grids being'adapted to be connected to anunbalanced source of signal potentials, two rectifier elements connectedin series-opposed relationship between the grids and forming anon-linearly conducting path from the cathode to the grid of each tubefor controlling the proportion of the total voltage of the sourceapplied to each tube to reduce the non-linear distortion produced byoperating the tubes with grid current.

VAN M. COUSINS.

The input circuits of the

